BM Guide 6683

A Guide to Polyolefin Blow Molding

z

H

Polyolefins for Blow Molding

H

C =C H

H

Figure 1. Ethylene monomer molecular structure

Polyolefins are the most widely used plastic for blow molding. This book, “A Guide to Polyolefin Blow Molding,” contains general information concerning materials, methods and equipment for producing high quality polyolefin blow molded products at optimum production rates. Blow-Moldable Polyolefins and Applications Polyolefins that can be blow molded include: z z z z

H H H H H H H H H H

C C C C C C C C C C H H H H H H H H H H Figure 2. Polyethylene molecular chain.

H

H

H H–C–H H

H

H H–C–H

C

C

C

C

C

H

H

H H–C–H H

z z

In general, the advantages of polyolefin blow molding resins are good processability, light weight, good toughness, outstanding chemical resistance and relatively low cost compared to other plastics. Furthermore, the basic properties of polyolefins can be modified to cover a broad range of end use properties. Polyolefins can also be coextruded with various other polymers, including ethylene vinyl alcohol (EVOH), nylon and tie-layers, to produce multilayer containers with improved barrier properties. Major application areas for polyolefin in blow molded products include: z

C H

z

H z

Figure 3. Polypropylene molecular chain.

Low density polyethylene (LDPE) Linear low density polyethylene (LLDPE) Medium density polyethylene (MDPE) High density polyethylene (HDPE) Ethylene copolymers, such as ethylene vinyl acetate (EVA) Polypropylene and propylene copolymers (PP)

z

z

Packaging for such products as milk and other foods, cleaning fluids, medicines, cosmetics and personal care products Automotive items, such as gas tanks, oil bottles and windshield fluid containers, air ducts and seat backs Consumer products, including toys, housewares and sporting goods Objects for materials handling, including 55-gallon drums and chemical carboys Industrial products, such as business machine fluid containers,

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Bellows-shaped shields and doublewall instrument and tool carrying cases.

This book contains extensive information on polyolefin blow molding; however, it makes no specific recommendations for the processing of Equistar Chemicals’ resins for specific applications. For more detailed information, please contact your Equistar polyolefins sales representative. Other Products from Equistar Equistar Chemicals offers an extensive range of polyolefin resins, plus polyolefin-based tie-layer resins not only for blow molding, but also for: z z z z z z z z z

Injection Molding Film Extrusion Extrusion Coating Sheet and Profile Extrusion Wire and Cable Coating Rotational Molding and Powder coating Blending and Compounding Flame Retardant Applications Pipe

Equistar also produces ethyl alcohol, ethyl ether, ethylene glycol and ethylene oxide. Information on all these products also can be obtained from your Equistar sales representative.

Polyolefins are Thermoplastics Derived from Petrochemicals Polyolefins are plastic resins polymerized from petroleum-based gases. The two principal gases are ethylene and propylene: Ethylene is the principal raw material for making polyethylene and ethylene copolymer resins; propylene is the main ingredient for making polypropylene and propylene copolymer resins. Polyolefin resins are classified as thermoplastics, which means that they can be melted, solidified and melted again. This contrasts with thermoset resins which, once molded, can not be reprocessed. Polyolefin resins for blow molding generally are produced in pellet form. The pellets are about one-eighth inch long and one-eighth inch in diameter and are usually somewhat translucent

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and water-white in color. Polyolefin resins sometimes will contain additives, such as thermal stabilizers. They can also be compounded with colorants, antistats, UV stabilizers, etc. to meet specific end-use needs. Effects of Molecular Structure The uses for polyolefin resins are primarily determined by three basic properties. These are: z z z

Average Molecular Weight Molecular Weight Distribution Crystallinity, or Density

These properties are essentially fixed by the catalyst technology and manufacturing process used to produce a specific grade of resin. The basic building blocks for the gases from which polyolefins are derived are hydrogen and carbon atoms. For polyethylene, these atoms are combined to form the ethylene monomer, CA, i.e., two carbon atoms and four hydrogen atoms (see Fig. 1). In the polymerization process, the double bond connecting the carbon atoms is broken. Under the right conditions, these bonds reform with other ethylene molecules to form long molecular chains (Fig. 2). The resulting product is polyethylene resin. For polypropylene, the hydrogen and carbon atoms are combined to form the propylene monomer, CH3CH:CH2, which has three carbon atoms and six hydrogen atoms (Fig. 3). The third carbon atom remains pendant and spirals regularly around the backbone chains. Ethylene copolymers, such as ethylene-vinyl acetate (EVA) are made by the polymerization of ethylene units with randomly distributed comonomer groups, such as vinyl acetate (VA). The polymerization of monomers creates a mixture of molecular chains of varying lengths. Some are short, others enormously long, containing several hundred thousand monomer units. For polyethylene, the ethylene chains have numerous side branches. For every 1,000 ethylene units in the molecular chain, there are about one to ten short or long branches. The branches radiate three-dimensionally (Fig. 4). Branching affects many polymer properties, including density, hardness, flexibility and transparency. Chain branches also become points in the

molecular network where oxidation may occur. In some processing techniques where high temperatures are reached, oxidation can adversely affect the polymer’s properties. Density Polyolefin blow molded products have a mixture of crystalline and amorphous areas. Molecular chains in crystalline areas are arranged somewhat parallel to each other. In amorphous areas, they are randomly arranged. This mixture of crystalline and amorphous regions (Fig. 5) is necessary for the proper balance of properties in good blow molded products. A totally amorphous polyolefin would be grease-like and have poor physical properties; a totally crystalline polymer would be very hard and brittle. HDPE resins have molecular chains with comparatively few side branches. Therefore, the molecular chains are packed more closely together. The result is crystallinity up to 95%. LDPE resins generally are 60 to 75% crystalline, while LLDPE resins’ crystallinity levels range from 60 to 85%. PP resins are highly crystalline, but they are not very dense. For polyethylene, the higher the crystallinity, the higher the resin density. Density is a primary indicator of the level of crystallinity in a polyethylene. Polyolefin blow molding resins have the following density ranges:

• • • • •

Figure 4. Polyethylene chain with side branches.

LLDPE resins have densities ranging from 0.910 to 0.940 grams per cubic centimeter (g/CM3). LDPE resins range from 0.916 to 0.925 (g/CM3) MDPE resins range from 0.926 to 0.940 (g/CM3) HDPE resins range from 0.941 to 0.965 (g/CM3) PP resins range from 0.890 to 0.905 (g/CM3)

EVA copolymers’ densities are functions of the proportion of vinyl acetate incorporated into the resin; as VA increases, density increases. The density of polyethylene resins influences numerous properties (Table 1). With increasing density, some properties, such as stiffness, are enhanced, while others, such as environmental stress crack resistance and low temperature toughness, are reduced toughness.

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Figure 5. Crystalline (A) and amorphous (B) regions in polyolefin.

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Table 1: General Guide to the Effects of Polyolefin Physical Properties on Resin Mechanical Properties and Processing.

Melt Index Increases

Density Increases

Chemical Resistance

Stays the Same

Increases

Clarity

Increases

Decreases

Elongation at Break

Decreases

Decreases

Flexibility

Stays the Same

Decreases

Gloss

Improves

Stays the Same

Heat Resistance (softening point)

Decreases

Increases

Impermeability to Gases/Liquids

Stays the Same

Increases

Low Temperature Flexibiity

Decreases

Decreases

Melt Viscosity

Decreases

Stays the Same

Mechanical Flex Life

Decreases

Decreases

Stress Crack Resistance

Decreases

Decreases

Tensile Strength at Break

Decreases

Increases

Figure 6.

The importance of good shear sensitivity—high viscosity in the parison, low viscosity through the die.

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Molecular Weight Atoms of different elements, such as carbon, hydrogen, etc., have different atomic weights. For carbon, the atomic weight is 12, and for hydrogen, it is 1. Thus, the molecular weight of the ethylene unit is the sum of its six atoms (2 carbon + 4 hydrogen) or 28. The molecular weight of an individual polymer molecule is the sum total of teh atomic weights of all combined ethylene units. Every polyolefin resin consists of a mixture of large and small chains, i.e., chains of high and low molecular weights. The molecular weight of the polymer chain generally is in the thousands. The average of these is called, quite appropriately, the average molecular weight. As average molecular weight increases, resin toughness increases. The same holds true for telongational properties and environmental stress crack resistance (cracking brought on when a molded part is subjected to stresses in the presence of solvents, oils, detergents, etc.). Melt Index Melt Index (for polyethylene)and Melt Flow Rates (for Polypropylene) are an indirect, simple measurement of the polymer’s average molecular weight. Polyethylene resin melt index is expressed in terms of the weight of flow through a standard orifice in a tenminute time period. This property is tested under standard conditions of temperature and pressure. Melt index (MI) is inversely related to the resin’s average molecular weight: as average molecular weight increases, MI decreases. Generally, a polyolefin resin with high molecular weight has a low MI, and vice versa. Melt index is an extremely important property since it describes both the flow of the molten polymer and many of the polymer’s end-use properties. The resin’s flow (or output in pounds/hour through an extruder) increases with increasing MI. Polyolefins with lower MI levels can sometimes require higher extrusion temperatures to make them flow easier. Conversely, most physial properties of the solid polymer are enhanced with lower melt index. Pressure can also influence flow properties. Two resins may have the same MI, but different high-pressure flow properties. MI data (Table 2) must therefore be used in conjunction with

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other properties, such as molecular weight distribution, to fully characterize the processability of a resin. Shear Sensitivity Melt index and density are two extremely useful numbers for describing a polyethylene resin; a third is “shear sensitivity.” The viscosity of melted HDPE appears to change depending on how rapidly it is undergoing shear, i.e, how fast it is being pushed through the extruder or die. Under moderate rates of shear, HDPE is moderately viscous. When it is forced at high speed through a narrow die gap, the HDPE melt is sheared at a fast rate; at this point it appears to have a low viscosity. Then, when it hangs from the die as a blow molding parison, under practically no shear, the same HDPE appears to be very viscous. All HDPE resins show varying degrees of shear sensitivity; in general, the faster the shear rate, the lower the apparent viscosity. HDPE resins with high shear sensitivity require less power and heat input to yield faster extrusion rates (Fig. 6). Molecular Weight Distribution The relative distribution of large, medium and small molecular chains in the polyolefin resin is important to its properties and especially to shear sensitivity. When the distribution is made up of chains close to the average length, the resin is said to have a “narrow molecular weight distribution” (Fig. 7). “Broad molecular weight distribution” polyolefins are those resins with a wider range of chain lengths. In general, resins with narrow molecular weight distributions will have greater stress crack resistance and better optical properties than resins with broad molecular weight distributions. Broader molecular weight distribution resins exhibit greater shear sensitivity, providing certain processing advantages in the blowmolding process. Comonomers Polyolefins made with one basic type of monomer are called homopolymers. Many polyolefins, called copolymers, consist of two or more monomers, each of which is called a “comonomer.” Many blow molding grades of LLDPE, LDPE, HDPE and PP incorporate varying types and amounts of comonomers, which provide specific property alterations.

The comonomers most often used with LLDPE and HDPE are called alpha olefins, and include butene, hexene and octene. Another comonomer used with ethylene to make blow molding grades is vinyl acetate (VA), yielding ethylene vinyl acetate (EVA). The incorporation of small amounts of VA with polyethylene results in a resin which extrudes similarly to polyethylene, but also provides tougher blow molded products with less stiffness and potentially greater clarity. A wide range of properties is possible depending upon the amount of VA incorporated and the synthesis conditions used to make the modified resins. Ethylene is the primary comonomer used with PP. PP random copolymers have scattered ethylene groups connected to the propylene backbone of the polymer. Such resins are distinguished by their superior clarity. PP impact copolymers have propylene backbones containing series of ethylene groups. These propylene copolymers are known for their medium to high impact strength at and below room temperature. Modifiers, Additives and Tie-Layers Numerous additives are commonly compounded with polyolefin blow molding resins (Table 3). In some grades thermal stabilizers, antistatic agents and nucleating agents are added during resin manufacture. Tie-layers are polyolefin-based resins that are used to bond one type of normally incompatible polymer with another during coextrusion. Tie-layers are specifically designed for use with such barrier materials as ethylene vinyl alcohol (EVOH), nylon (PA), polyester (PET) and polyvinylidene chloride (PVdC).

Blow Molding Resins Available from Equistar Equistar offers a wide range of polyolefin resins for blow molding, including PETROTHENE® , LLDPE, LDPE, HDPE and PP, ALATHON® HDPE, FLEXATHENE® TPO, and ULTRATHENE® EVA copolymers. These resins can be tailored to meet the requirements of many applications. Some typical specialty grades are:

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Table 2: Polyolefin General Purpose Blow Molding Resins/ Melt Indices Resin

Melt Index Ranges* g/10 min.

LLDPE LDPE HDPE EVA PP